1. Field of the Invention
The present invention relates to a honeycomb structure, a honeycomb structure aggregate, and a honeycomb catalyst.
2. Description of the Related Art
In the related art, a honeycomb catalyst, which includes a medium carried by a honeycomb structure and is used for conversion of exhaust gas from vehicles, is fabricated by disposing materials of high specific surface area like active alumina and catalysts like platinum on the surface of a monolithic cordierite-based honeycomb structure. In addition, the honeycomb catalyst carries an alkali-earth metal like barium to act as an occlusion-type NOx catalyst for processing NOx in an oxygen-rich atmosphere, such as atmospheres in a lean-burn engine or a Diesel engine.
In order to further improve conversion performance, it is necessary to increase the contact probability between the exhaust gas and the catalyst element and the NOx occlusion agent as well. For this purpose, it is required to increase the specific surface area of the carrier, decrease diameters of catalyst elements and highly disperse the catalyst elements. However, if simply increasing the amount of the materials of high specific surface areas like active alumina, it is the thickness of the active alumina layer only that is increased, and it does not lead to an increase of the contact probability, but results in a pressure loss. For this reason, modifications have been made on cell shapes, cell densities, wall thicknesses, and so on. For example, see JP-A 10-263416 (below, referred to as “reference 1”).
On the other hand, a honeycomb structure formed from materials of high specific surface areas is known to be formed by extrusion molding of the materials of high specific surface areas together with inorganic fibers and inorganic binders. For example, see JP-A 5-213681 (below, referred to as “reference 2”).
Further, a honeycomb structure is known, which is obtained by extrusion molding of the materials of high specific surface areas and having a catalyst being carried by the materials. In the honeycomb structure, the thickness of a partition wall (referred to as wall thickness, below) between through-holes, the length along a gas flowing direction, the length of one side of the through-holes, and an open area ratio are adjusted to be preset values. For example, see JP-A 2003-245547 (below, referred to as “reference 3”).
Additionally, the entire contents of JP-A 10-263416, JP-A 5-213681, and JP-A 2003-245547 are hereby incorporated by reference.